Endoprosthesis with long-term stability

ABSTRACT

The invention relates to the use of an active substance complex for creating biological parts, in particular organs for living organisms, with the following components which differ from one another and are specifically adapted to the respective biological part which is to be created, namely at least one structural component based on extracellular material specifically adapted to the cells of the respective biological part which is to be created, at least one recruiting component, at least one adhesion component, and at least one growth and/or maturation component for producing an endoprosthesis implant. 
     The outer surface (I) of the endoprosthesis is coated at least partially with the active substance complex. In addition, the endoprosthesis has at least one cavity (II) which is filled with the active substance complex. 
     In order to reduce the amount of active substance complex needed for filling the at least one cavity (II), the active substance complex can additionally be applied to a further support material such as collagen or a suitable polymer.

The invention relates to the use of an active substance complex forcreating biological parts, in particular organs for living organisms,with the following components which differ from one another and arespecifically adapted to the respective biological part which is to becreated, namely at least one structural component based on extracellularmaterial specifically adapted to the cells of the respective biologicalpart which is to be created, at least one recruiting component, at leastone adhesion component, and at least one growth and/or maturationcomponent.

An active substance complex for creating biological parts, in particularorgans for living organisms, with said components is already known inthe prior art.

This object is attained by a complex active ingredient comprising astructure component, at least one recruiting component, at least oneadherence component, and at least one growth and/or maturationcomponent, preferably in the form of at least one cytokine.

That known active substance complex has the quality of passing overcells with a reciprocal reaction and inducing them to form a biologicalpart. For this purpose the active substance complex (implant), which canalso be performed on an industrial scale, for instance in the frameworkof series manufacture, is to be produced outside the body of the livingorganism and then brought into contact with cells which are to form thebiological part. This can occur at a suitable site to which the activesubstance complex is introduced, which can actually be inside the bodyof the living organism, but can also be outside the body, for instancein a cell culture. In doing this, the active substance complex accordingto the invention is brought together with an accumulation of vital,function-capable and specific cells at the desired site for formation ofthe biological part.

As is known, biological parts generally consist of specific cells andextracellular material produced by the cell, but only the cellularportions have their own metabolic activity. Since the active substancecomplex according to the invention arranges everything for theproduction of the signals required for the biological part, it is nowpossible to hold the cells required for this purpose at the site of theactive substance complex to the desired geometry, to increase theirnumber and to mature them with a view to the desired functions. Becausethe active substance complex for the production of biological partscontains the suitable relevant component for any required partialformation step, its production is guaranteed in its entirety.

Furthermore, with use of the active substance complex according to theinvention, same-body cells can be used for production of the biologicalpart, so that the known difficulties which arise with the otherwisetraditional transplantations are overcome. Especially, transmission ofillness is no longer possible, and likewise no long-termimmunosuppression with its grave side effects is required, and theindividual living organism remains a genetically uniform entity.

Biological parts take up a certain amount of space for their functionalperformance. Frequently their function is connected with a certaingeometry within they fulfill their function. This is also true for thebiological parts produced by means of the active substance complex ofthe present invention. The active substance complex used for productionof a biological part fulfills this function with the aid of a structurecomponent which one the one hand exerts the space-retaining function andon the other hand allows the assumption of the existence of a geometricform within which the biological part which is produced fulfills itsfunction.

In one preferred embodiment, the active substance complex of the presentinvention is primarily a macromolecular three-dimensional matrix, whichtogether with water and salt can be present in the form of a gel ofdistinct expansion properties. Thus, for instance, proteoglycan gels mayform the matrix. A network of fibers, such as, for instance, differenttypes of collagens, or elastin, can also form the structure component.Likewise, combinations of gels with intercalated fibers are suitablecomposite materials. The structure component for the production ofbiological parts is manufactured differently for the different intendeduses, so that it can be used as a fleece, a gel or a liquid gel, whichcan be cut, milled, or be plastically deformed or cast.

The structure component is adapted to the requirements of the biologicalpart to be produced, since a certain specificity exists between cellularand the extracellular portions of biological parts. Sources for theproduction of the structure component are therefore primarilyextracellular materials of different tissues or organs. For instance,for the production of skin, or for the production of the structurecomponent, cutaneous proteoglycan and fiber proteins are used; for theproduction of the spleen, spleen-specific proteoglycans and fibrousproteins are used; for the production of bone, bone-specificproteoglycan and fibrous proteins are used: etc. The structure componentcan also include metallic, ceramic, vitreous, polymeric or fatty carriermaterials, to aid in the modification of the geometric, mechanical,chemical or special properties of the structure component. Thus, thecarrier material together with the structure component can be present insolid, porous, membranous, micell, viscous or liquid form according tothe requirements, which are determined for the production of thebiological part and its subsequent function.

In another preferred embodiment of the active substance complexaccording to the invention, the material displays its capacity forproduction of the biological parts essentially only temporarily. Inother words, the active substance complex is configured so that it iscyclically controllably decomposable and following the production of thebiological part is then no longer even present. The rate ofdecomposition of the active substance complex can thus be assumed bydifferent transverse cross-linking of the polymeric matrix and/or theaddition of (enzyme) inhibitors and/or immunosuppressive and/orinflammation-inhibiting materials. The inhibitors claimed in thiswriting can be low-molecular compounds which occupy the active center ofthe decomposing enzyme but they can also be chelating agents, which bindan essential cofactor of the enzyme to themselves, or to neutralizingantibodies. Other types of inhibiting mechanisms are possible.

As inflammation-inhibiting and/or immunosuppressive additives, thefollowing can be used: inhibitors of the phospholipase, such as, forinstance, steroids, inhibitors of cyclooxygenase, such as, for instance,indomethacin inhibitors of the lipozygenase, such as, for instance,nordihydroguaiaretic acid, immunosuppressives of the type includingcyclosporine and/or of the type including anithymocytene-globulin, etc.

According to this invention, to produce biological parts, living cellsof the desired type are to be collected in the region of the structurecomponent. For this purpose the structure component of the activesubstance complex includes one or more recruiting components with theaid of which the desired cells are stimulated to move in a certaindirection. Chemotactica (chemotaxines) are suitable for use asrecruiting component(s).

The chemotactica suitable for this use have been described for a numberof cells and can be isolated from human, animal, plant or microbialsources of even be produced by chemical synthesis or biotechnicalmethods. If the structure component projected outside the body of theliving organism is introduced with its recruiting component(s) into anorganism and/or is brought into contact with target cells outside theorganism, it then builds a concentration gradient, in which the targetcells are oriented, whereby the relevant recruiting component correlateswith the specific identification or recognition structures on the targetcells, which are characterized as receptors. For the case wherein thebiological part to be produced is composed of a plurality of types ofcells, the structure component, corresponding to the number of types ofcells, includes a plurality of recruiting components in the form ofchemotactica.

The specificity of the relevant recruiting component of the differenttarget cells as well as the amount of chemotactic activity isascertained by research, wherein the directed migration of the desiredcells through defined filter pores is measured under the effect of acertain gradient of the chemotacticum in a chamber. The activeingredient system can be biologically standardized relative to itsrelevant recruiting component by means of researched techniques of thissort, which is important for the industrial production of the activesubstance complex.

Peptides for instance such as N-F-met-leu-phe and/or for instancemetabolites of arachidonic acid, such as leukotrienes, with the aid ofwhich certain cells can be attracted out of the blood, or phagocytes,will serve as chemotactica. Proteins, such as for instance a proteinwhich attracts mesenchym cells, work chemotactically especially onconnective tissue cells.

In addition to the specificity of the recruiting component for thedesired target cells and the amount of chemotactic activity, the timeduration of the activity during which the chemotactic concentrationgradient is built up is also specific and is of considerable length.This kinetic is adapted to the requirements for production of biologicalparts by the active substance complex according to the invention bymeans of a controllable liberation of the relevant recruiting componentfrom the structure component. In doing this at this point, the rate ofdecomposition of the structure component plays a role, as does also thetype of connection between the structure component and the relevantrecruiting component, dependent for instance on whether it has to dowith a covalent or an associative linkage. With covalent linkage, slowersynthesis and longer maintenance of the chemotactic gradients isattained than with merely associative linkage made up of ionic forces orhydrogen bridge linkage. The recruiting of the cells for production ofthe biological part however for the most part occurs more rapidly thanthe decomposition of the structure component, since the infused cellsare quite essential for decomposition of the proteoglycen/collagenmaterial.

For production of the biological parts, following infusion of the cellsinto the structure component, these cells are in turn to be fixed at thesite of the structure component, in order to prevent its emigration intothe environment and to guarantee a stable architecture of the biologicalpart which is produced. For this purpose the active substance complex,includes one or more adherence components, by means of which the infusedcells can be fixed at the site of the structure component. Thus theadherence components “anchor” themselves on the one hand to the cellsbeing built up on the biological parts and on the other hand to themacromolecular network of the structure component. Adhesins which areknown as having a certain anchoring specificity include proteins such asfibronectin or laminin, with the aid of which connective tissue cells orepithelial cells can be anchored to the structure component. Numerousother adherence factors of different specificity can be made availableand come into use according to the biological part to be produced withthe active substance complex according to the invention. Among others,cell adherence molecules L-CAM and N-CAM belong to this group; theadherence molecules cytotactin, tenas, laminin, fibronectin, collagentypes IV, V, VIII, as well as synthetic peptides, and the partialsequences of different adhesions represent the matrix, andtransmemebrane protein compounds, such as for instance integrin.

To increase the specificity of attachment of the desired cells to thestructure component during production of the biological parts,antibodies inhibiting undesired adherence components can be introduced.The biological activity of the adherence components can be measured inadherence tests of various types (e.g. by means of centrifugal forces,etc.) and thus can be standardized for the entire active substancecomplex.

Frequently the cells are fixed by suitable adhesions and chemotacticallyattracted to the area of the structure component for production of thebiological part are insufficient in number to constitute the biologicalpart. Also, the mobile cells available in an organism for this processare found for the most part in an insufficiently mature state to fulfillall of the functions of a biological part, wherein they frequentlyrepresent precursors or parent cells out of which the function-capable,mature cells of the biological part to be produced must then develop.For this purpose the active substance complex according to the inventionhas at least one growth and/or

maturation component, preferably in the form of one or more cytokines,under the effect of which the number of infused cells is increased andalso a maturation of the cells occurs.

Cytokines are materials of distinct chemical structure which arecharacterized in that they cooperate in reciprocal reaction with cellsand influence the splitting and growth of cells as well as theirmaturation and biosynthesis. Cytokines thus have a hormone-like effectbut do not display this quality in the presence of hormones from adistance but rather only in localized areas, which is advantageous inthe production of biological parts, since this is a localized process.

A great number of different cytokines of different specificity areknown. These can be used to influence cell growth, differentiation andmaturation and also to influence the metabolism of the infused cellswhich are introduced as other components in the active ingredient systemaccording to the invention. The specificity of the cytokines for certaincells is determined by the presence of corresponding receptors on thetarget cells, whereby the interaction of a cytokine with the receptortriggers the resulting cellular reaction. The receptors on the targetcells describes in this case are found in membrane proteins, which passinto reciprocal reaction with the chemotacticum which is being used,link with it and invade the cell. With recycling of the receptors theyare again available for linkage with chemotacticum. Analogously, withthe receptors for the cytokines being introduced, it has only to do witha different specificity, while with the same reciprocal reactionmechanism. While the linkage of the chemotacticum leads to directedmovement of the target cells, the linkage of the cytokines to thecorresponding receptor of the target cell results in growth and/ordifferentiation. Frequently the receptors are not yet characterizedmolecularly, so that they are known only by their specificity for therelevant ligands (chemotacticum, cytokine, etc.)

Therefore, it is to be taken into account that not infrequentlystimulating or inhibiting sequential

processes can be triggered at the cells, according to the specificity ofthe relevant cytokine and target cells. The desired cellular reaction ofthe cytokine in terms of reciprocal reaction for the production ofbiological parts is generally connected with a dual signal transmission,so that inan advantageous manner at least two cytokines are used in the activesubstance complex according to the invention, in order to attain bothgrowth and differentiation. Following interaction with a cytokine, manycells produce more cytokines and release them, whereby the cellsthemselves can thus be stimulated or inhibited (the so-called autokrinermechanism). Frequently the specificity of the cells for certaincytokines is modified with individual differentiation steps, so that nolonger can any interaction occur, or even the reciprocal reactions of asequential reaction can change over from a stimulating to an inhibitingcellular reaction. The properties of a number of cytokines are known, sothat the cytokine effect can likewise be standardized in the activeingredient system.

Some examples of cytokines, which for instance, function in theproduction of blood, are the factors stimulating colonies, there being,in the production of connective tissue the fibroblasts growth factor, inthe production of skin the epidermal growth factor, in the production ofcartilage the cartilage-inducing factor, in the production of spleen orlymph nodes the lymphocytes-activating factor as well as spleen peptide,for the production of thymus the T-cells growth factor as well as thymuspeptide, for the production of bone the bone growth factor as well asthe transforming growth factor, for the production of blood vessels theanglogenesis factor. Furthermore, the following cytokines are also used:interleukins, growth factors similar to insulin, tumor necrosis factor,prostaglandins, leukotrins transforming growth factors, growth factorderiving from thrombocytes, interferons, as well as growth factorsderiving from endothelial cells.

Since biological parts are composed most often of a plurality of celltypes, combinations can occur. Thus for instance the formation of bloodvessels is important for blood supply to the biological part beingproduced, so that accelerated vessel-formation comes into questions interms of addition of an anglogenesis factor as cytokine component of theactive ingredient system. Similarly, accelerated formation of nerveconnections can be important, and can be realized by a correspondingintroduction of additional cytokines into the active substance complex.

It was an object of the present invention to make the active substancecomplex available for wider use.

This object is achieved by the use of said active substance complex tocreate an endoprosthesis implant. Compared with the conventionally usedendprostheses not having the active substance complex, this permitslong-term stabilizing of the endoprosthesis. In this use according tothe invention, an active substance complex is used which is suitable forcreating biological parts in the form of bones and has the followingcomponents which differ from one another and are specifically adapted tocreating bone, namely at least one structural component based onextracellular material specifically adapted to the cells of the bonewhich is to be created, at least one recruiting component, at least oneadhesion component, and at least one growth and/or maturation component.

The discovery of this use according to the invention was the result ofextensive studies on combining the active substance complex withdifferent support materials, in particular metal support materials. Thecombination of a support material with the active substance complex isnot unproblematic. Based on previous experience of the active substancecomplex and of its complex mode of action, one would have to expect atleast a reduced formation or recreation of the particular biologicalpart to be treated, for example osseous regeneration. The risk of ahistotoxic reaction has also been suspected.

The solution to this object was therefore not obvious since, as hasalready been explained, it is extremely problematic to combine theactive substance complex and a support, in this case an endoprosthesis,because the functions of the active substance complex in the bone defectcould then be disturbed or at least complicated by possible immunereactions.

The endoprosthesis to be stabilized has an outer surface which is coatedat least partially with the active substance complex and/or it has atleast one cavity which is filled with the active substance complex.

This coating and/or filling with the active substance complex isintended to permit more rapid and permanent incorporation of theendoprosthesis in the organism. Accelerated and at the same timeimproved incorporation of the endoprosthesis at the implanted siteresults in longer-term stability and in greater and earlier loadabilityof the endoprosthesis.

According to a further embodiment, the endoprosthesis has at least onecavity which is filled with the active substance complex, the activesubstance complex additionally being applied to a further supportmaterial. Collagen or a suitable polymer can be used as such a furthersupport material. The collagens of types I, IV, V and VII are mentionedhere in particular. The collagens can be used for example in the form ofwebs or gels and they in particular have an inherently goodimmunological compatibility and are easy to process.

The polymer support materials which can be used are in particularpolymers of natural monomers, such as polyamino acids (polylysin,polyglutamic acid, etc.), and polymers of lactic acid. Copolymers canalso be used, for example of polylactic acid and hydroxyacetic acid.

Polylactates are polyesters of lactic acid having the chemical formula:

Direct polymerization of the monomers results in polymers withrelatively low molecular weights. The upper limit is about 20 000 Da.Higher molecular weights can result by linking of cyclic dimers at hightemperature and low pressure and in the presence of catalysts. Lacticacid polymers are biodegradable, biocompatible, insoluble in water, andcharacterized by a high degree of strength.

The additional use of a further support material such as collagen or thestated polymers reduces the amount of active substance complex needed tocompletely fill the cavity of the endoprosthesis, without adverselyaffecting the basic efficiency of the active substance complex. In thisway, the use of the active substance complex is made more economicallyfavorable.

The invention also relates to an endoprosthesis which is coated with orwhich comprises the active substance complex in one of its embodimentsaccording to the use.

The invention is explained in greater detail below on the basis ofexamples and with reference to the attached drawing, in which:

FIG. 1 shows a diagrammatic representation of new bone formation inrabbits using the active substance complex, compared with an untreatedsample,

FIG. 2 shows a diagrammatic representation of new bone formation insheep using the active substance complex, with tricalcium phosphate assupport material, compared with pure tricalcium phosphate,

FIG. 3 shows a diagrammatic representation of new bone formation in ratsusing the active substance complex, with different collagens as supportmaterial, compared with pure collagens,

FIG. 4 a shows a side view of an endoprosthesis used for coating withthe active substance complex, and

FIG. 4 b shows a further side view of the endoprosthesis, turned through90° compared with FIG. 4 a.

I. PREPARATION OF THE ACTIVE SUBSTANCE COMPLEX

The main steps in the preparation of the active substance complex aredescribed below: Tubular bones from calves, sheep, rabbits or rats werecleaned, the bone marrow, inter alia, was removed, and the bones werethen frozen. The frozen bone was ground to a particle size of less than2 mm. The ground bone pieces were defatted in acetone and decalcified in0.6 N hydrochloric acid. The product was then freeze-dried and ademineralized bone matrix was obtained which was extracted in 4 molarguanidium-HCl solution. The extraction solution was dialyzed againstdistilled water and the active substance complex was obtained bycentrifuging off and freeze-drying in the precipitate.

This basic method of preparation is shown below as a flow chart.

II. Efficacy of the Active Substance Complex Without Use of SupportMaterials

To show that the active substance complex is effective per se, a test isfirst set out in which the active substance complex is implanted withoutadditional support materials.

1. Animals Used in the Test

Female chinchilla rabbits with a mean bodyweight of 3089 g were used.They received a rabbit maintenance diet and double-ozonized tap wateracidified with hydrochloric acid to pH 4.5 ad libitum.

The animals were anaesthetized by subcutaneous injection of a mixture ofketamine and xylazine.

2. Preparation of a Bone Defect in the Rabbits

An internally cooled drill was used to prepare an implant bed of 4 mmdiameter and circa 9 mm depth in the knee joint (distal end of femur) ofthe rabbit. The bore hole thus formed was then filled in each case withand 90 mg of the active substance complex which had been produced asdescribed under I. A further bore hole in each case was left untreatedand served as a control for new bone formation.

FIG. 1 shows the new bone formation in the untreated hole and in thebore hole after implantation of the active substance complex and alsothe density of the surrounding pre-existing spongy substance 28 daysafter the operation (n=2/active substance quantity).

Analysis of the tests revealed that the density of the spongy substancesurrounding the bore holes after implantation of 30 mg of the activesubstance complex was 45% higher than in the untreated hole, and, afterimplantation of 90 mg of the active substance complex, was 69% higherthan in the untreated hole. The quantity of pre-existing spongysubstance had no influence at all on the regeneration in the defectbecause the new bone formation after insertion of the active substancecomplex did not start from the periphery of the bore hole but insteadwas distributed uniformly across the defect.

III. Bone Formation in the Mandible of Sheep Using Tricalcium Phosphate(TCP)

Tricalcium phosphate (TCP) is a calcium phosphate ceramic based on theCaO/P₂O₅ system and is prepared by pressing and subsequently sinteringthe starting materials calcium oxide (CaO) and diphosphorus pentoxide(P₂O₅). Alternatively, it can also be prepared in a hot-pressing step.

1. Animals Used for the Tests

Fully grown domestic sheep from Viehzentrale Südwest AG of Stuttgartwere used in the tests described below. They were supplied with hay andwater and, three days before the operation, a slurry of Altrominpellets.

The animals were premedicated with 1 ml xylazine/1 ml ketanest i.m. Thesheep were then anesthetized with Nembutal.

2. Preparation of the Implant

TCP was suspended in a solution of 100 mg of dissolved active substancecomplex with 10 ml of water and deep-frozen with liquid nitrogen underconstant stirring. After 24 hours of freeze-drying and subsequent gassterilization (ethylene oxide), the TCP thus doped with the activesubstance complex was introduced into the mandibular defect describedbelow in a sheep. In addition, a further mandibular defect serving forcomparison purposes was filled with undoped TCP sterilized in anautoclave.

3. Preparation of the Mandibular Defect in Sheep

A sheep mandible was suitably prepared and, with physiological salinesolution as coolant, a trephine of 5 mm diameter was used to cut out andremove in each case a standardized cylinder of bone. One of the boreholes thus formed was then filled with TCP, which had been doped withthe active substance complex according to test procedure 1, and thesecond bore hole was filled with undoped TCP.

For purposes of clarity, the results of the bone growth in themandibular defects are shown in graph form in FIG. 2. The test durationwas 26 days and 41 days respectively.

It was found that doping TCP with the active substance complexaccelerated bone regeneration of the mandibular defect in both sheep No.811 and No. 86 by about 100% in the initial phase. After 41 days, therate of acceleration of bone regeneration was still 10%. Bone healing istherefore much more rapid, particularly at the start, than it is withoutthe osteoproductive effect of the implants doped with the activesubstance complex.

This finding is of importance particularly for coating endoprostheseswith the active substance complex. An endoprosthesis coated with theactive substance complex, for example in the case of a fracture of theneck of the femur, accordingly permits more rapid incorporation of theprosthesis and thus more rapid regeneration and recovery of therespective patient. The length of the hospital stay is thereforeshortened.

IV. Tests with Collagens as Support Materials

The already known active substance complex discussed above can be usedfor the incorporation of endoprostheses. In the production of the activesubstance complex, the quantitative yield at the required degree ofpurity is very low. We therefore examined whether there are supportmaterials which can be combined with the active substance complex so asto reduce the quantity of active substance complex needed for theparticular objective, but without thereby reducing its bone-formingefficiency.

1. Active Substance Complex

The active substance complex used for the purposes of the testsdescribed below was prepared exactly in the manner described under I.,using tubular bones from calves.

2. Animals Used in the Tests

Male Wistar rats weighing between 350 and 400 g were used and were keptin an air-conditioned animal housing at 23° C. and about 50% relativehumidity. They were given a maintenance diet for rats and mice.

Two implants of the same support material were introduced into theabdominal musculature of each test animal, of which one implant wascoated with the active substance complex while the other remaineduncoated and served as a comparison implant. The animals were sacrificedafter 21 days, and the affected areas of the implants in the abdominalmusculature were explanted and histologically evaluated.

3. Support Materials Used

In these tests, collagen materials were used which are all commerciallyavailable. Collagen A was a pure, sterile, native, resorbable bovineskin collagen, free from any foreign additives such as stabilizers ordisinfectants.

Collagen B was a purified, freeze-dried, lightly cross-linked sterileand nonpyrogenic bovine skin collagen with weakly antigenic properties.The helical structure of the collagen was preserved.

Collagen C comprised pure, native and resorbable bovine collagenfibrils.

All the collagens used were in web form. Collagen web sections each of50 mg were cut out, and 1 ml of the active substance complex solution (3mg/ml) was added in each case. In the control implants, 1 ml ofdistilled water was added instead. The collagen web sections thustreated were frozen at −20° C., freeze-dried and yielded implants with adiameter of about 10 mm and a thickness of about 5 mm. FIG. 3 shows thebone formation results for collagen implants A, B and C inimmunosuppressed animals and non-immunosuppressed animals after 21 days,with and without coating with the active substance complex (cyclosporinA). Here, the evaluation figure (BZ) corresponds to the arithmetic meanof the evaluation figures from three independent persons on six implantsof each group.

Collagen A coated with the active substance complex showed a boneformation effect in immunosuppressed animals after this period of time,whereas this could not be demonstrated for collagen B. By contrast,however, collagen C showed a very pronounced bone formation effect.

It follows from this that it depends on the preparation of theparticular collagen used and this dictates its suitability as a supportmaterial. Collagens which are immunogenic are not suitable for use assupport materials.

IV. Testing Support Materials for Their Biocompatibility

In tests relating to the improvement of the long-term stability ofendoprostheses, titanium disks of different surface roughness (100, 20and 0.5 μm), a TiAl₆V₄ alloy (0.5 μm) and Al₂O₃ disks from the companyFriedrichsfeld and hydroxylapatite disks from Feldmühle AG were used.Hydroxylapatite is obtained by ceramic firing of pentacalciumhydroxidetriphosphate powder at 1250° C. In addition, a hydroxylapatite ceramiccan also be produced using a natural material such as the carbonateskeleton of red alga. After a washing and drying procedure, the organicconstituents are first removed by pyrolysis at a temperature of about700° C. This is followed by conversion to hydroxylapatite by addition ofphosphate solution at elevated pressure and increased temperature.

In a further method for producing a hydroxylapatite ceramic, startingfrom the natural skeleton of corals, the calcium carbonate of the coralsis converted by hydrothermal conversion to hydroxylapatite or a mixtureof hydroxylapatite and other mineral structures. In the material thusobtained, the coralline structure, i.e. in particular theinterconnecting pore system of the coral, is preserved.

The coatings with the active substance complex, which had been preparedusing the general procedure set out above, were applied by thedip-coating method. Dip-coating is understood as a coating method inwhich the object to be coated, in this case the disks, is dipped into asolution with a desired predetermined concentration of the coatingagent, in this case the active substance complex. This is followed byfreeze-drying. Thin cover layers or coatings are obtained. The testingof the specified materials for their biocompatibility was carried out inparticular with reference to the surface roughness (n=20; four diskseach).

This biocompatibility testing of the materials under investigationrevealed that titanium, with the highest number of living cells and thebest ratio of living cells to dead cells, is very well suited as asupport material. While hydroxylapatite provided a similarly goodresult, TiAl₆V₄ was considerably poorer.

Generally, as regards surface roughnesses, it was found that thesmoothest surfaces, i.e. surfaces with a pore diameter of 0.2–0.5 μm,yielded the best results, with the exception of TiAl₆V₄. As theroughness or pore diameter increases, the number of living cells andalso the ratio of living cells to dead cells drop. The highestproportion of living (bone) tissue in direct contact with the disksurface was obtained with a pore diameter of about 0.5 μm.

TABLE 1 Number of living Number of dead Support material cells per cm²cells per cm² Hydroxyl apatite 0.2–0.5 μm 1792 ± 700  200 ± 37  20 μm7469 ± 2614 2238 ± 715  50 μm 4477 ± 408  1692 ± 427  Osprovit 7930 ±2007 1638 ± 377  (Feldmühle) Titanium 0.5 μm 11377 ± 2538  1054 ± 308 20 μm 9600 ± 3038 1754 ± 439  100 μm 2308 ± 669  2085 ± 623  TiAl₆V₄ 0.5μm 7200 ± 1062 2800 ± 954  Al₂O₃, extra pure, 11446 ± 1500  2292 ± 600 polished

The results of these tests could now be carried over to the coating ofendoprostheses with the active substance complex. A view of theendoprosthesis used is shown in FIG. 4.

Before the endoprostheses were used, their outer surface (I) was coatedwith the active substance complex by the dip-coating process, and theactive substance complex was additionally introduced into the innercavities of the prosthesis stem (II) which have openings on the stemsurface. This has the advantage, in the event of possible futureloosening of the endoprostheses, that the active substance complex canbe applied subsequently without any great effort and leads to boneformation and thus to stabilizing of the endoprosthesis. If so desired,a coating with the active substance complex can also be provided in thearea of the screw connection (III).

The fact that coating with the active substance complex leads to higherloading capacities compared with uncoated surfaces is illustrated inTable 2, using the example of hydroxylapatite (HA). The tensile strengthvalues at the interface between different implant materials weredetermined in N/mm²±standard deviation. Hydroxylapatite prepared by hotisostatic pressing (HIP) was compared with hydroxylapatite additionallycoated with the active substance complex. The implant material wasimplanted in the distal area of the femur of rabbits and examined after84 days. The tensile strength values found are set out in the followingtable.

TABLE 2 Material SR (μm) Days n Tensile strength HA HIP 0.5 84 10 1.53 ±0.24 HA HIP AS 0.5 84 6 2.27 ± 0.31 N = number of implants AS = coatingwith active substance complex HIP = hot isostatic pressing SR = surfaceroughness

Finally, it must be pointed out again that the tests carried out for thepurposes of the present invention are all carefully designed modeltests, because the actual subject, i.e. the endoprosthesis implanted forexample in the area of the femoral bone, could not be made available forthe tests, as it would not have been acceptable to conduct these testson the human body.

In addition, the invention can be applied to all conceivableendoprostheses. The description of the example of the endoprosthesis inthe area of the neck of the femur is illustrative in character.

1. An endoprosthesis implant having an outer surface with pores, saidpores having a pore diameter of 0.2–0.5 μm which is partially coatedwith an active substance complex, said active substance complexcomprising the following components derived from bone: at least onestructural component comprising a macromolecular three-dimensionalmatrix derived from bone-specific proteins; at least one recruitingcomponent comprising a chemotactic material for recruiting bone growthcells to said matrix; at least one adhesion component for adhering saidcells to said matrix; and at least one growth or maturation component.2. The endoprosthesis implant of claim 1, wherein the surface includesat least one cavity which is filled with the active substance complex.3. An endoprosthesis implant having at least one cavity with an outersurface inside the cavity with a diameter of 0.2–0.5 μm which is filledwith an active substance complex, said active substance complexcomprising the following components derived from bone: at least onestructural component comprising a macromolecular three-dimensionalmatrix derived from bone-specific proteins; at least one recruitingcomponent comprising a chemotactic material for recruiting bone growthcells to said matrix; at least one adhesion component for adhering saidcells to said matrix; and at least one growth or maturation component.4. The endoprosthesis implant of claim 1, wherein the structuralcomponent is selected from the group consisting of collagen, elastin orproteoglycans.
 5. The endoprosthesis implant of claim 1, wherein therecruiting component is selected from the group consisting of at leastone chemotactic peptide or metabolite of arachidonic acid.
 6. Theendoprosthesis implant of claim 1, wherein the adhesion component isselected from the group consisting of fibronectin, tenascin, laminincollagen types IV, V, VII, L-CAM, N-CAM or integrin.
 7. Theendoprosthesis implant of claim 1, wherein the growth and maturationcomponent is selected from the group consisting of cytokines.
 8. Theendoprosthesis implant of claim 1, wherein the active substance complexfurther comprises a support material selected from the group consistingof non-immunogenic collagen and polymers of natural monomers.
 9. Theendoprosthesis implant of claim 3, wherein the structural component isselected from the group consisting of collagen, elastin orproteoglycans.
 10. The endoprosthesis implant of claim 3, wherein therecruiting component is selected from the group consisting of at leastone chemotactic peptide or metabolite of arachidonic acid.
 11. Theendoprosthesis implant of claim 3, wherein the adhesion component isselected from the group consisting of fibronectin, tenascin, laminin,collagen types IV, V, VII, L-CAM, N-CAM or integrin.
 12. Theendoprosthesis implant of claim 3, wherein the growth and maturationcomponent is selected form the group consisting of cytokines.
 13. Theendoprosthesis implant of claim 3, wherein the active substance complexfurther comprises a support material selected from the group consistingof non-immunogenic collagen and polymers of natural monomers.